1. Field of Invention
The present invention relates to a back light module and a liquid crystal display. More particularly, the present invention relates to a back light module that provides a surface light source with a high brightness level to illuminate a liquid crystal display.
2. Description of Related Art
With the rapid advance in semiconductor technologies and man-machine interface designs, multi-media are at the forefront of communication. In a multi-media world, displays are an important device. Although cathode ray tube (CRT) is economical to produce and has advantages in many aspects, possible production of hazardous radiation, a bulky body and enormous power consumption are three major factors that diminish its desirableness in a multi-terminal desktop environment. With a better resolution, space utilization and power consumption, liquid crystal display (LCD) has been developed to serve as a substitute for the CRT.
FIG. 1 is a schematic cross-sectional view of a conventional liquid crystal display. As shown in FIG. 1, the liquid crystal display 100 mainly comprises a liquid crystal panel 110 and a back light module 120. The back light module 120 is assembled to the backside of the liquid crystal panel 110. The back light module 120 furthermore comprises a light-guiding plate 122, a side light source 124, a reflective holder 126 and a frame 128.
The light-guiding plate 122 is a wedge-shaped panel with a top surface 130, a bottom surface 132 and a side surface 134. The top surface 130 is a light-emitting surface, the bottom surface 132 is a light-scattering surface and the side surface 134 is a light-incident surface.
The side light source 124 is a cold cathode fluorescent lamp (CCFL) positioned next to the side surface 134 of the light-guiding plate 122. Light from the side light source 124 enters the light-guiding plate 122 through the light-incident surface 134. After scattering and reflection at the light-diffusion surface 132, the light travels to the light-emitting surface 130. In other words, the light-emitting surface 134 provides a surface light source for illuminating the liquid crystal panel 110.
The reflective holder 126 is positioned next to the light-incident surface 134 of the light-guiding plate 122 and the side light source 124 is enclosed within the reflective holder 126. Light from the side light source 124 is collected upon the side surface 134 of the light-guiding plate 122 after traveling to the reflective holder 126. The frame 128 supports the light-guiding plate 122, the side light source 124 and the reflective holder 126 so that the light-guiding plate 122, the side light source 124 and the reflective holder 126 together form an integrative unit.
Since the liquid crystal molecules will not emit any light by themselves, the back light module 120 must be used to provide a surface light source for illuminating the liquid crystal panel 110 to a sufficiently high level of brightness and contrast.
At present, liquid crystal displays have progressed from the XGA type to ones having a higher level of resolution. Because a higher level of resolution is now limited by the aperature of liquid crystal panel, resolution can be increased only through an increase in the brightness level. In general, the brightness level of a liquid crystal display can be enhanced through an increase in the brightness of the side light source. However, most back light modules using a cold cathode fluorescent lamp to serve as the side light source have limited capacity for brightness increase through an increase in the tube current. Moreover, increasing the tube current not only increases overall power consumption, but also leads to the shortening of the working life of the lamp inside the back light module.
Aside from boosting up the lamp current, other method of increasing the brightness level of the liquid crystal display includes increasing the number of lamps in the back light module. However, increasing the number of lamps not only increases the power consumption, but also increases the overall weight and thickness of the back light module. In other words, the back light module can no longer be streamlined when the additional lamps are incorporated. Moreover, the inverter for driving the back light module must be re-designed as the number of lamps is increased and effort to protect electrical devices against electromagnetic interference must be multiplied.